High speed impact line printers comprise a continuously moving type carrier such as an endless belt or band having engraved type characters movable at constant speed parallel to a row of print hammers. A print medium such as a continuous paper web and ink ribbon are located between the hammers and the type carrier. The print hammers are uniformly spaced parallel with type carrier to define a print line. The spacing of the hammers corresponds to the desired spacing of the printed characters when printed and generally has one print hammer for each print position. In some applications, the number of print hammers may be fewer than the number of print positions and the hammers could span more than one print position between hammers. In either case, the hammers are separated by an interhammer clearance gap and characters are printed by the hammers at predetermined positions. As the print force is confined within the width of the hammers, printing cannot be made at the gaps between the hammers. This limits band printers from use in applications such as bar code printing where it may be desirable to produce imprints within the region of the interhammer gaps. The problem is due to the fact that the print force is essentially confined within the boundaries determined by the hammer structure whereas the code patterns include portions which must occur within the region of the interhammer clearance gaps.
A particular instance in which the problem occurs is when vertical bar code, such as UPC, is to be printed. Vertical bar code patterns comprise combinations of horizontally spaced vertical bars of different widths separated by spaces of different widths. The combination of thick and thin bars with thick or thin spaces forms the code by which the data characters are recognized by a bar code reader. There is no problem in printing vertical bar code when the bar imprints are within the boundaries of the hammer as in the case of printing alphanumeric characters. However, the bar code formats currently in use require bar imprints to be formed to some extent in the region of the interhammer clearance gap. In those instances, the bar must be impacted when located outside the normal boundaries of the hammers. This produces an incomplete bar imprint which affects the dimensions of the bar code imprint which can produce errors in detection and decoding by the scanner devices which read the code. The problem also occurs when printing horizontal bar code with horizontal bars vertically spaced where the type element is wider than the boundary of the hammers and gaps are formed in the horizontal bar imprint.
Printing vertical bar code with a high speed impact line printer is known. U.S Pat. Nos. 4,027,586 and 4,762,063 show printing vertical bar code using a type carrier which is a rotating drum. An article of R. L. Gilbert published in the IBM Technical Disclosure Bulletin, Vol. 17, No. 5, pp. 1320 et seq. also shows vertical bar code printing using a drum printer. An article of A. H. Ett published in the IBM Technical Disclosure Bulletin, March 1981, Vol. 23, No. 10, pp. 4739 et seq. shows a bar code design which avoids the problem of closely spaced bar imprints caused by variation in the flight time of the print hammers of a chain or train printer. None of this art deals with the problem of printing bar code at locations within the interhammer clearance gaps of the print hammers.